[SI-LIST] Re: Traces don't cause EMI - really?

  • From: "Brent DeWitt" <bdewitt@xxxxxxxxxxxxx>
  • To: <leeritchey@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Sun, 19 Oct 2003 21:27:58 -0600

It seems this has become something of a tempest in a teacup.  As I mentioned
in an earlier post, I believe much of the energy of the dispute has
developed over differences in terminology.

First, in the vast majority of cases, I agree with Mr Ritchey.  In the
twenty five years or so I've been involved in EMC, I've never seen the
radiation from  surface trace fail a product.  That said, I've only worked
on old/slow boards with fundamental clocks less than 1.5 GHz, although I've
worked with 900 MHz intentional radiators to their 10th harmonic.  Rules of
thumb only work until the thumb is too fat to see the problem underneath it.
As I mentioned in an earlier post, resonant patch antennas and other
intentional radiating pcb structures are nothing more than fat, well
designed traces.  Somewhere between them and our ideal EMC designs lies
practicality.  As frequency increases, I find myself needing increasingly
closer inspection of detail.

In somewhat oblique support of Mr. Ritchey, most failing EMC issues I've
observed have been associated with L di/dt induced voltages on the reference
plane caused by surface traces.  I/O cables, using said poorly controlled
reference planes out to the world, are often a major emissions issue, but
that is entirely another subject for discussion.

Finally,  I believe Mr. Ritchey is correct, but using the following
assumptions:

- The trace structure and geometry does not approach a resonant structure at
the fundamental or appreciable harmonics of the operating frequency.
- The reference plane structure supporting the return currents of the
surface trace does not significantly contribute to reference plane resonance
and induced voltage on attached cables.

Respectfully,

Brent DeWitt

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Lee Ritchey
Sent: Sunday, October 19, 2003 6:06 PM
To: Larry Barnes; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Traces don't cause EMI - really?


I downloaded a copy of this paper a few minutes ago.  The conclusions of
the paper do not state that microstrip traces emit significant EMI.  All
that is stated is that the analysis and chamber measurements agree within
measurement accuracy of what little energy is detected.  The actual amount
is not stated in the paper.  There is certainly no claim that the detected
emissions are large enough to cause concern among EMC engineers.

This is likely one of those cases where the difference is between
detectable and significant.  There is no claim in this paper that the
emissions are significant, only detectable.

If we are to accept that surface traces are significant sources of EMI,
there needs to be credible demonstration of this.  The reason is, designers
are expected to expend cost avoiding microstrip traces.  To do this, there
needs to be demonstrated emissions that are worth the extra cost to control.

My challenge to the proponents of this claim is to provide this
demonstration.

I and many others have designed hundreds, maybe thousands of PCBs with
traces on outer layers with all the products successfully passing all
emissions tests.  That could not happen if these claims are true.

Lee


> [Original Message]
> From: Larry Barnes <larry.barnes@xxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 10/18/2003 5:42:14 AM
> Subject: [SI-LIST] Re: Traces don't cause EMI - really?
>
> I just couldn't resist after all this discussion.  I would refer you to =
> a study done by Dr. Dave Hill of NIST.  I demonstrates that microstrips =
> indeed radiate.  The title of the paper is "Radiated Emissions and =
> Immunity of Microstrip Transmission Lines: Theory and Reverberation =
> Chamber Measurements"  IEEE Trans on EMC  May 1996.=20
>
> Larry
>
>
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> Lawrence C. Barnes
> QLogic Corporation
> Aliso Viejo, CA  92656
>
>
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